Progress has been made in the purification and characterization of complement components and in studies of their mechanism of interaction. We have shown that intact C3 does not bind to receptors, while C3b does bind. These findings have been made possible by the new methods of purification of complement components which leave them in the native rather than altered state. The regulation of biologically active fragments of C3 by cells and their products has been demonstrated. Major progress has also been made in the study of complement-bacterial interactions. It has been shown that human anti-pneumococcal antibody previously thought not to interact with complement does so and provides a measure of host defense. Failure of patients with splenectomy to synthesize antibody following pneumovax vaccination was documented. This required development of a new antibody assay procedure. The factors that contribute to the removal of bacteria from the bloodstream were documented in molecular terms. Anti-cell wall antibodies bind to bacteria and deposit complement molecules on the bacterial surface but provide no measure of host defense. Anti-pneumococcal capsule antibodies functioning in the same way provide specific increases in host defense capacity. The difference between susceptible and resistant bacteria to the lytic action of complement has been clarified.